The invention relates to correction of phase and amplitude imbalance caused by an I/Q modulator in a transmitter.
In new radio systems, the scarcity of radio frequencies makes it necessary to use spectrum-efficient modulation methods. In Europe, a new radio system standard has been developed for PMR (Professional Mobile Radio) users, called TETRA (Terrestrial Trunked Radio). xcfx80/4-DQPSK (xcfx80/4-shifted Differential Quadrature Phase Shift Keying) has been selected as the modulation method of the system. As far as a transmitter is concerned, a drawback to the modulation method is the variation in the amplitude of the envelope of a radio frequency signal, which causes InterModulation (IM) in a nonlinear amplifier. The IM results spread the spectrum of a transmitted signal and thus tend to reduce the gain obtained from using the linear modulation method. The IM results cannot usually be filtered since they are formed extremely close to the desired signal. With constant-amplitude modulation methods, no spreading of the spectrum occurs; therefore, the signal can be amplified by a nonlinear amplifier.
A trunked PMR system, wherein different user groups share the same radio channels, has stringent requirements regarding adjacent channel interference caused by a transmitter. These requirements necessitate good linearity in the transmitter of the radio system used.
In a power amplifier, good linearity is only achieved with poor efficiency. However, the efficiency of portable equipment should be as high as possible for the operation time to be sufficient and in order not to waste battery capacity. In addition, at least relatively good efficiency is required of power amplifiers at base stations in order to avoid cooling problems. Sufficient efficiency and linearity can only be achieved by linearizing the transmitter.
If the nonlinearities of an amplifier were known in advance, it would be possible to form inverse functions of the nonlinearities to convert the input signal, whereby the nonlinearities would be cancelled. The characteristics of the amplifier do not, however, stay the same but they change due to, for example, aging, warming up, and according to the radio channel and power level used. In addition, amplifiers have individual differences. Linearization methods are needed that are capable of adjusting adaptively to changing conditions. Research has been conducted on many different linearization methods, and three have been found to possess characteristics suitable for practical radio systems. These methods are feedforward, Cartesian feedback and predistortion. A linearization method can also be adaptive.
Thus, if the nonlinear transfer function of the amplifier is known and if it does not vary as a function of time, the signal to be transmitted can be linearized by applying to the signal a suitable transfer function causing predistortion. Hence, the signal outputted from the amplifier can be made linear. This method is called predistortion. At baseband, for example, predistortion can be carried out by using a lookup table (LUT) into which are stored conversion parameters causing predistortion, i.e. predistortion parameters, in which case the conversion parameters to be used at a given time are selected on the basis of the amplitude of the signal to be predistorted.
In a predistortion system based on amplitude, distortion of the amplitude of a signal is assumed to be independent of the phase of the signal. An I/Q modulator in the transmitter, however, causes distortions dependent on the phase of the signal, which impoverish the performance of the predistortion system. An I/Q modulator operates on a quadrature modulation basis. It enables two independent signals to be combined in the transmitter and be transmitted on the same transmission band and the signals to be separated again at the receiver. The principle of quadrature modulation is that two separate signals, I and Q (Inphase and Quadrature phase), are modulated by using the same carrier wave frequency, but the phases of the carrier waves differ from each other in that the carrier wave of signal Q lags 90xc2x0 the carrier wave of signal I. After modulation, the signals are summed. Thanks to the phase difference, the I- and Q-signals can be separated from each other when the sum signal is demodulated. The distortions cause errors in the mutual phase and amplitude balance of the I- and Q-signals. Typically, the amplitude imbalance caused by the I/Q modulator is few per cents (2 to 5%) and the angle error 2 to 3 degrees. Correction circuits with fixed correction parameters set therein have been used in connection with predistortion systems for correcting the distortion caused by the I/Q modulator. The problem is then manufacturability since each I/Q modulator requires unique parameters which must be set as early as during the production. In addition, the magnitude of the phase and amplitude imbalance caused by the I/Q modulator depends on the frequency, in which case when the frequency is changed, the fixedly-set correction does not necessarily work any longer. Another known solution has been disclosed in xe2x80x9cNew Methods for Adaptation of Quadrature Modulators and Demodulators in Amplifier Linearization Circuitsxe2x80x9d by Cavers, J. K. in IEEE Transactions on Vehicular Technology, Vol. 46, No. 3, August 1997, pp. 707 to 716. It discloses a system wherein the correction parameters are determined by comparing the output of an I/Q modulator with the input thereof. The solution requires, however, an additional feedback branch independent of the feedback branch of the predistorter, which results in a complex arrangement.
An object of the invention is thus to provide a method and an arrangement implementing the method such that the above-mentioned problems can be solved. The objects of the invention are achieved by a method of determining correction parameters used for correcting the phase and amplitude imbalance of an I/Q modulator in a transmitter, which transmitter comprises an I/Q modulator for I/Q-modulating a signal to be transmitted and a corrector for correcting the phase and amplitude imbalance caused by the I/Q modulator according to the correction parameters, which method comprises the steps of feeding an I- and Q-test signal into the transmitter, and sampling the I/Q-modulated test signal to be transmitted, the method being characterized by further comprising the steps of A/D-converting the signal samples taken from the test signal to be transmitted, I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, determining the phase and amplitude imbalance caused by the I/Q modulator on the basis of the I- and Q-feedback signals, and determining the correction parameters of phase and amplitude on the basis of the determined phase and amplitude imbalance.
The invention is based on the idea that the I/Q demodulation of the feedback signal is carried out digitally, whereby the I/Q demodulation does not cause phase or amplitude imbalance for the signal. The phase and amplitude imbalance caused by the I/Q modulator, and thus the correction parameters of correction feedback, can then be determined by means of the feedback signals since the feedback signals comprise phase and amplitude errors caused only by the I/Q modulator. An advantage of the method of the invention is that the feedback arrangement is simple. Furthermore, the same feedback branch can be used for determining the parameters of the predistorter and the corrector for the I/Q modulator. This considerably simplifies the structure of the predistortion system of the transmitter and, as fewer components are needed, also saves costs.
The invention further relates to an arrangement for determining correction parameters used for correcting phase and amplitude imbalance of an I/Q modulator in a transmitter, which transmitter comprises an I/Q modulator for I/Q-modulating a signal to be transmitted and a corrector for correcting the phase and amplitude imbalance caused by the I/Q modulator according to the correction parameters, which arrangement comprises means for sampling the I/Q-modulated test signal to be transmitted, which is formed from I- and Q-test signals fed into the transmitter, the arrangement being characterized by further comprising means for A/D-converting the signal samples taken from the test signal to be transmitted, means for I/Q-demodulating the signal samples digitally into I- and Q-feedback signals, means for determining the phase and amplitude imbalance caused by the I/Q modulator on the basis of the I- and Q-feedback signals, and means for determining the correction parameters of phase and amplitude on the basis of the determined phase and amplitude imbalance. Such an arrangement enables the advantages of the method of the invention to be achieved by a simple structure.
Preferred embodiments of the invention are disclosed in the dependent claims.